1. Field of the Invention
This invention relates to a variable magnification optical apparatus such as a variable magnification copying apparatus in which an image of an original can be formed on a photosensitive surface at selectively different magnifications.
2. Description of the Prior Art
In the above-described optical apparatus, when the imaging magnification is to be changed, the ratio of the length of the optical path between an original and a lens to the length of the optical path between the lens and a photosensitive surface is changed correspondingly to a selected magnification. It is usually the case that the position of the lens is changed correspondingly to the selected magnification to change the ratio of the lengths of the optical paths.
Now, it is usually the case with the actual lens that due to various factors during the manufacture thereof, the focal length thereof differs from the design value (nominal focal length). That is, the focal length of the actual lens includes an error relative to the design value. (The difference between the design value and the actual focal length will hereinafter be referred to as the error .DELTA.f of the focal length ) Accordingly, the position of the lens and the length of optical path during one-to-one magnification image formation are corrected correspondingly to the error of the focal length of the lens so that a focused one-to-one magnification image may be formed on the photosensitive surface, but if the lens is subsequently moved over a distance as per the design value to form a reduced image or an enlarged image, the image on the photosensitive surface will be out of focus due to the error of the focal length.
As known literatures, there are U.S. Pat. Nos. 3,259,009; 3,416,860; 3,431,053 and 4,155,641. Any of these patents discloses a device for adjusting the position of the lens or the mirror, but none of them suggests the technique of changing the imaging magnification. Thus, these known literatures merely disclose the technique of focusing an image in an optical apparatus for forming only an image of one magnification. U.S. Pat. No. 4,077,715 discloses a technique whereby, in a variable magnification optical apparatus, the amount of movement of the mirror during original image magnification change is corrected correspondingly to the error of the focal length of the lens. However, in the apparatus of this U.S. Pat. No. 4,077,715, by said correction of the amount of movement of the mirror, the length of the optical path between the original and the lens is corrected correspondingly to the error of the focal length of the lens, but the length of the optical path between the lens and the photosensitive surface is not corrected and therefore, it is impossible to more accurately focus the image of the original formed on the photosensitive surface and it is also difficult to make the magnification of the image of the original more approximate to the target magnification.